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Bathymetry

The development of bathymetry charts requires the correction of the tide on hydrographic soundings. This operation is called reduction of soundings.

Definition of the reduction of hydrographic soundings

The reduction of hydrographic soundings consists of reducing the depths measured by the sounder to an arbitrary but precisely defined depth, to which the hydrographic datum, also called chart datum, can be attached with sufficient accuracy for the purposes of navigation. This operation consists of solving two problems:

determine the chart datum in a known reference system, depending on the geographical position;

determine, with respect to this reference system, the sea level which varies with both location and the time of the sounding.

These problems are largely independent (even though the values come from the same measurements and the same processing - in particular, traditional and spatial methods are both based on tide analysis and prediction) and can be solved using different techniques.

Reduction of soundings and tide gauging

To reduce soundings, the necessary measures were long provided by portable float-type tide gauges placed within sight of the coast. They have the disadvantage of requiring a fixed vertical structure, which is often incompatible with the proximity of the sounding area. In addition, the measurements acquired are too often of poor quality.

Pneumatic bubbler tide gauges were sometimes used for this purpose, resulting in fewer restrictions in terms of site selection.

However, during hydrographic missions, these instruments are not always used with the due care needed to obtain accurate measurements.

For this application, pressure sensor systems (submerged and resting on the bottom) called "bottom tide gauges" are increasingly being used. They are linked to the levelling network by a few readings of a tide staff placed nearby if possible. For soundings further from the coast, these gauges are also used to measure the tide in the sector. However, to obtain the water level, the measurements from these sensors must be corrected for atmospheric pressure and density of seawater. Moreover, these devices cannot provide real-time measurements, which might be useful both for using the data and checking sensor operation. Though there are prototypes that can transmit data upon request, there is a need for a robust (for easy transport), high quality instrument that can send real-time measurements electronically.